System and method for measuring hole orientation for SPECT collimators

ABSTRACT

A method for performing reconstruction of single photon emission computed tomographic images wherein forward and/or backward projection steps in the reconstruction utilize measured collimator hole orientation angles, whereby the reconstructed tomographic images have improved image resolution and reduced distortion and artifact content.

RELATED APPLICATIONS

This is a continuation-in-part (CIP) application of the U.S. patentapplication Ser. No. 12/987,376, filed on Jan. 10, 2011, the disclosureof which is hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The disclosure generally relates to the field of image reconstruction,and more particularly, to a system and method for compensating forinaccuracies in collimator hole geometries to provide reduced distortionand improved resolution of tomographic images.

BACKGROUND

Single Photon Emission Computed Tomography (SPECT) reconstructs threedimensional images of radioactive source distributions in the body usinga sequence of planar images acquired over a range of angles around thepatient.

In order to reconstruct the tomographic image from the set of planarimages it is necessary to know the direction from which the photonsdetected at a given point in the image originated. For X-ray computedtomography (CT), the direction is defined by a line from the anode tothe X-ray detector. For Positron Emission Tomography (PET), it is a linebetween the pair of detectors in which the two coincident 511 keVphotons are detected. In SPECT, the direction is usually defined by acollimator—a lead plate with 20,000 to 50,000 small holes formed in itwhich restricts the detected incident photons to only those with knownangles of incidence at the detector.

The most popular collimator type is the parallel beam collimator, inwhich the holes are designed to point perpendicular to the detectorsurface. Another type of collimator is referred to as a fan beamcollimator. With the fan beam collimator, the holes in one dimension(transverse) focus to a point; there is no focusing in the axialdimension. A further type of collimator is referred to as a cone beamcollimator. The cone beam collimator focuses to a single point in bothtransverse and axial dimensions.

Reconstruction algorithms in the current state of the art assume thatthe construction of these collimators is perfect. Such algorithmsperform back projection of planar projection data and forwardprojections of the object estimates under this assumption. In reality,however, collimators are not perfect. Their construction is subject todimensional errors such that all holes do not point in the idealintended direction. This leads to errors in forward and backwardprojections and, results in distortions and degradation of theresolution in the final tomographic images. Inconsistencies between thephysical, imperfect collimator and the idealized collimator modelutilized in the forward and back projection steps of the tomographicreconstruction lead to artifacts in the tomographic images.

Therefore, there is a need for an improved method for accounting forinaccuracies in the collimator hole pointing directions, and foremploying this accounting in the reconstruction process to removedistortions and improve the resolution of reconstructed images.

SUMMARY

The disclosed system and method improve the quality of reconstructedimages by performing forward and/or back projections using a vector mapof the hole directions at each point of the collimator surface. Thisapproach will work for arbitrary collimation geometry and willautomatically account for errors in the collimator “pointing vectors,”thereby minimizing distortions and improving reconstructed imageresolution.

An exemplary method is disclosed for mapping the hole directions overthe entire surface of the collimator. The technique is applicable toarbitrary collimator geometries. A specific example is given for themulti focal collimator—a variable focal length fan in two independentdimensions—however, it will be appreciated that the system and methodcan be used with any collimator type.

According to an implementation of the present disclosure, a method isprovided for reconstruction of single photon emission computedtomographic images wherein forward and/or backward projection proceduresin the reconstruction utilizes actual measured SPECT collimator holeorientation angles, whereby the reconstructed tomographic images haveimproved image resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of the invention, both as to its structure and operation,may be obtained by a review of the accompanying drawings, in which likereference numerals refer to like parts, and in which:

FIG. 1A shows a schematic representation of an arrangement fordetermining collimator hole angle using scanning radioactive linesources;

FIG. 1B shows a top-down schematic view of a collimator and a scanningradioactive line source illustrating the orientation and scanningdirection of the line source with respect to the collimator.

FIG. 2 is a schematic illustration of a carrier holding a set of aplurality of line radiation sources;

FIG. 3A shows a series of line images representing a series of thereference positions X₁ in the x-axis direction in the collimator's x-ycoordinate plane;

FIG. 3B shows a series of line images representing a series of themeasured positions X₂ in the x-axis direction in the collimator's x-ycoordinate plane;

FIG. 4 is a schematic illustration of a dual-headed SPECT detectorsystem that can be utilized to implement the arrangement 10 of FIG. 1A;

FIG. 5 is a flowchart describing a method of determining collimator holeangle using scanning nuclear line sources;

FIG. 6A shows a system in which the collimator holes and theforward-/backprojection steps during tomographic reconstruction conformto the idealized, mathematically perfect collimator model; and

FIG. 6B shows a system in which the collimator holes directions differfrom the ideal, and where their actual measured directions are utilizedin the forward-/backprojection steps of tomographic reconstruction, inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1A shows a schematic of an arrangement 10 for use in measuring thehole angles of a collimator 8. The arrangement 10 allows measuring thecollimator hole angles using one or more line radiation sources 2. Themeasured collimator hole angles are then compiled into a vector mapproviding the orientation of the holes in the collimator 8 that can beincorporated into the forward and/or backward projection steps oftomographic reconstruction in SPECT imaging. Knowing the actualorientation of the collimator holes rather than assuming that they areoriented in the intended ideal orientation makes the forward andbackward projection process more accurate.

The method of measuring the orientation of the collimator holes will nowbe described in conjunction with the FIGS. 1A-6. Referring to FIG. 1A, aline radiation source 2 is positioned a known distance D above theinteraction plane 4 of a medical imaging detector 6. A collimator 8,whose hole angles are to be measured, e.g. a multi-focal lengthcollimator, is positioned between the line radiation source 2 and thedetector 6. The line radiation source 2 is configured and adapted to becontrollably movable or scanned in a direction A parallel to theinteraction plane 4 of the detector 6 so that the line radiation source2 maintains its distance D above the detector's interaction plane 4.

FIG. 1A shows only one line radiation source 2 viewed from one end ofthe line radiation source 2 for purpose of simplifying the description.In practical application, however, for matter of efficiency, a pluralityof line radiation sources are used to minimize the time required to makethe measurements. For example, FIG. 2 shows a carrier 14 on which isprovided a plurality of the line radiation sources 2. The plurality ofline radiation sources 2 are in parallel arrangement and they are a setdistance W apart. The absolute value of the spacing distance W isarbitrary but it is a fixed value for a given set of line radiationsources 2. The line radiation sources 2 are filled with an appropriateisotope. In one embodiment, a total of twenty line radiation sourcesfilled with technetium isotope are used. It will be appreciated thatother isotopes, and a greater or lesser number of line sources may alsobe used.

FIG. 1B shows a schematic top-down view of the arrangement 10 showing aline radiation source 2 above the collimator 8. The collimator 8 is a2-dimensional array of holes that extend in parallel rows in the x and ydirections noted in the drawing. The holes' locations are defined in anx-y coordinate as shown and the line radiation source 2 is configured tobe parallel to one of the x or y axes of the collimator 8 and movable indirection A that is perpendicular to the line radiation source. In theillustrated example, the line radiation source 2 is parallel to the yaxis and is being scanned along the direction A which is orthogonal tothe y axis and parallel to the x axis. Thus, the view shown in FIG. 1Ashows the arrangement 10 along the x axis.

Using the illustration of FIG. 1A, measuring the orientation angle ofone hole in the collimator 8 along the x axis will be described. In theillustrated example, the collimator 8 is a variable focal length fanbeam collimator as represented by the hole orientation lines 7. Theholes of the collimator 8 are configured to form an approximately fanbeam region 7 a and variable focal length regions 7 b.

First, an image of the line radiation source 2 is taken using thedetector 6 through the variable focal length fan beam collimator 8. Thedetector will see the line radiation source 2 through a set of detectorpixels located near X₂ on the detector 6 which have a direct line ofsight to the line radiation source 2 through the collimator 8. By this,we mean that a set of detector pixels detect the radiation beam 22 fromthe line radiation source 2 through a set of collimator holes that arealigned between the line radiation source 2 and the detector pixels nearthe position X₂. We refer to sets of detector pixels near the positionX₂ rather than a single detector pixel because collimator holes atmultiple positions X₂ can view, at least partially, the source 2,leading to a distribution of energized pixels with peaked energizedpixels at X₂. Thus, the image of the line radiation source 2 is detectedby the detector pixels near position X₂ that are displaced by a distancedx from a reference position X₁ that is directly under the lineradiation source 2. The position X₂ will be referred to herein as themeasured position.

Next, another image of the line radiation source 2 is taken using thedetector 6 through a parallel hole reference collimator (not shown).Because the holes in the parallel hole reference collimator are orientedorthogonal to the plane of the collimator, the detector will see theline radiation source 2 through a set of detector pixels located nearthe reference position X₁ by detecting the radiation beams travelingalong the line 21 and impinging on the detector pixels. Thus, bycomparing the two line images, the offset distance dx between thereference position X₁ and the measured position X₂ can be measured.

Because the line radiation source 2 is always at a distance D from thedetector's surface 4, as illustrated in FIG. 1A, we see that thearrangement of the line radiation source 2, X₁ and X₂ form a righttriangle. And because D and dx are known, the angle θx is related to Dby tan θx=dx/D and θx can be calculated using the formula:θx=tan⁻¹(dx/D)   (1)

One of ordinary skill in the art would readily understand that the orderin which the two line images are taken is not important. In other words,the line image for the reference collimator can be taken first or afterthe line image for the collimator 8 being measured. By repeating thesteps described above along the y axis of the collimator 8, the angularorientation of the collimator hole in the y direction can be measured.

One of ordinary skill in the art would readily understand that theaccuracy of the measured hole angle θx is dependent upon the accuracy ofthe collimator holes in the parallel hole reference collimator. However,there is a class of parallel hole collimators, whose holes are all aimedsubstantially orthogonal to the collimator surface. These collimatorstend to have much better dimensional accuracy and the holes are aimedtrue to their intended direction and the inventors have found that theydo not substantially affect the accuracy of the measured hole angle θx.In one embodiment, the parallel hole reference collimator would have adimensional tolerance of less than ±0.12 degrees.

In order to measure the orientation angle of all of the collimator holesalong the x axis, one can capture images of the line radiation source 2through all of the collimator holes along the x axis by scanning theline radiation source 2 in multiple steps across the width of thecollimator 8/detector 6 assembly along the x axis and take images ateach stepped position 2′. This scanning/stepping movement of the lineradiation source 2 relative to the collimator 8/detector 6 assembly andthe detector can be accomplished using an X-Y stage. Preferably anautomated programmable X-Y stage would be used to enable the relativemovement between the line radiation source 2 and the collimator8/detector 6 assembly. In the example where a parallel hole referencecollimator is used for determining the location of the referenceposition X₁, the scanning/stepping would need to be performed once withthe parallel hole reference collimator and once with the collimator 8being measured. However, because a collimator generally has hundreds ofholes across its width, this process can take a long time using one lineradiation source.

Thus, in a practical application of the method, a plurality of lineradiation sources as shown in FIG. 2 can be used. FIG. 2 shows a carrier14 holding a set of a plurality of line radiation sources 2 positionedin parallel relation and separated by a fixed distance W. According to apreferred embodiment, the line radiation sources 2 are provided insufficient number to cover the width of the collimator 8. For purpose ofdiscussion, we use an example where 20 line radiation sources that arespaced 2 cm apart on the carrier 14 sufficiently spans the width of thecollimator 8. In this embodiment, the carrier 14 is scanned across thewidth of the collimator 8 in 2 mm stepped increments. At 2 mmincrements, a total of ten discrete steps are all that is necessary toscan the whole width of the collimator 8.

At each stepped position, an image of the plurality of line radiationsources 2 is taken using the detector 6 similar to the single lineradiation source example discussed above in connection with FIG. 1A.Thus, a set of images of the line radiation sources 2 are producedcorresponding to each stepped position. These images will be referred toherein as “line images.” At each discrete stepped position of theplurality of line sources 2 during this scanning/stepping, themechanical position (in the x-y coordinate of the collimator 8) of theline sources 2 and the line images obtained by the detector 6 are storedin a suitable storage medium provided in the controller system that iscarrying out the scanning. Similar to the single line radiation sourceexample, the scanning/stepping procedure producing the set of lineimages is performed once with the collimator 8 being measured and, inthe preferred embodiment and, again with a parallel hole referencecollimator. The two scanning/stepping procedures, one with the referencecollimator and the other with the collimator being measured, are carriedout so that the mechanical positions for the plurality of line radiationsources 2 at each of the stepped interval are same in bothscanning/stepping procedures. That is, the crucial requirement is thatthe spatial locations of the lines in both acquisition steps arerepeatable. Alternatively, in the case of a single collimatoracquisition, the reference positions X₁ of each line must be known apriori in terms of the position setting of the automated programmableX-Y stage scanning the lines over the detector.

The scanning/stepping performed with the parallel hole referencecollimator generates a set of line images representing a series of thereference positions X₁. The scanning/stepping performed with thecollimator 8 generates a set of line images representing a series of themeasured positions X₂. FIG. 3A shows an example of the series of lineimages 16 representing a series of the reference positions X₁. FIG. 3Bshows an example of the series of line images 18 representing a seriesof the measured positions X₂. In other words, FIG. 3A is a series ofimages of the intensities of collimated radiation impinging on thedetector 6 for the parallel hole reference collimator, while FIG. 3B isthe corresponding series of images of the intensities of collimatedradiation impinging on the detector 6 for the variable focal length fancollimator 8 positioned at the same locations over the detector. Theline images (a)-(d) in both sets of images represent the images taken atfirst four of the ten stepped intervals. In this embodiment, going fromleft-to-right, the entire carrier 14 holding the plurality of lineradiation sources is translated by two millimeters per step, such that,after ten steps the collimator 8 has been sampled in two millimeterincrements in the x axis direction.

The distance dx is obtained by noting the difference in the positions ofrespective lines in the two series of images. However, because each linein the line images are produced by multiple rows of detector pixels,each line is a raw image of ˜10-15 pixels wide (along x axis) havingapproximately Gaussian, bi-Gaussian profile along the x axis directionfor the parallel and variable fan collimators, respectively. Therefore,the position of the lines (X₁ and X₂) along the x axis direction at agiven detector pixel position in the y axis direction must be determinedby finding the “peak” of the imaged line's intensity profile in the xdirection (“x-profile”). At each line's y axis position (along they-dimension or vertical in the images of FIGS. 3A and 3B), the peak ofthe imaged line's x-profile can be determined by standard peak locatingtechniques. These include fitting a Gaussian/bi-Gaussian curve to theline's x-profile, or other peak location metrics.

In an alternate embodiment, the reference position X₁ can be determinedwithout the use of a reference collimator. The reference position X₁ canbe determined from the automated programmable X-Y stage being used forthe scanning/stepping motion of the line radiation source 2 relative tothe collimator 8/detector 6 assembly. For example, by calibrating theprogrammable X-Y stage's position relative to the collimator 8/detector6 assembly or ensuring that the dimensional tolerances of the allcomponents of the X-Y stage, collimator holder, and the detector, etc.the reference position X₁ can be determined from the position of theautomated programmable X-Y stage itself. This alternate embodiment willbe referred to hereinafter as the single collimator acquisition sincethe reference collimator is not needed.

After the dx values for each of the line images in FIGS. 3A and 3B arecalculated, the collimator hole angles θx along the x axis direction canbe calculated using the equation (1) discussed above. It would beobvious to one or ordinary skill in the art that the collimator holeangles θy along the y axis direction for the variable focal length fanbeam collimator 8 can be calculated by repeating the scanning/steppingprocess described above in the y axis direction using the parallel holereference collimator and the variable focal length fan beam collimator8. That process would generate another set of series of line imagessimilar to those shown in FIGS. 3A and 3B, from which dy values(analogous to dx values) can be calculated and then the orientationangles θy along the y axis direction can be calculated using theformula:θy=tan⁻¹(dy/D).

The scanning/stepping process for the y axis direction would be carriedout by rotating the carrier 14 by 90 degrees so that the plurality ofline radiation sources 2 are now oriented orthogonal to the orientationshown in FIG. 1B. This will allow the plurality of line radiationsources 2 to be scanned/stepped across the collimator 8 along its widthin the y axis direction. (See FIG. 1B for the x-y axis orientation).

Again, in the alternate single collimator acquisition embodiment, thereference positions Y₁ necessary for determining the offset distance dyin the y-direction can be determined without the use of a referencecollimator.

The offset distance dx data from x-axis scan and dy from the y-axis scanare combined to generate a data set, vector map, of the collimator holesin the collimator 8. The vector map would include the θx and θy valuesfor each hole of the collimator 8. The vector map is stored as part ofthe firmware associated with the particular collimator 8 so that aparticular SPECT system in which the collimator 8 is installed, theSPECT system would be able to utilize the collimator's vector map datato accurately perform forward and/or back projection in reconstructingthe SPECT image.

FIG. 4 is an example of a system 50 that can be utilized to implementthe arrangement 10 of FIG. 1A. In this example, the system 50 is a SPECTsystem comprising at least a patient bed 51, detector units 52, 53, agantry 55 providing support for the detector units 52, 53 and a controlunit 57. The patient bed 51 is configured and adapted to be controllablymovable in axial directions L, as well as vertical direction V. Thepatient bed 51 is used to hold the patient during the normal operationof the SPECT system, however, in this embodiment, the SPECT system 50 isutilized for the method described herein and the patient bed 51 isutilized as the movable stage on which the carrier 14 is mounted forperforming the scanning/stepping procedure. The axial movement of thepatient bed 51 in the direction L includes the directions along the yaxes in the collimator's x-y coordinate plane. The scanning/steppingprocedures in the x and y directions described above can be achieved byrotating the carrier 14 by 90° on the patient bed 51. The distance D iscontrolled by the patient bed's movement in the vertical direction V anddetector head 53 radius. The detector unit 53 generally comprises acollimator 58 and a detector 6. The operation of the system 50 and themovement of the patient bed and the detector unit 53 are controlled bythe control unit 57. The control unit 57 is provided with appropriateprocessor units 60, machine-readable memory units 62, and user interfaceunits 64 for proper functioning of the system 50.

According to the alternate embodiment described above, a dedicated X-Ystage can be used to scan the line sources/carrier over a detector whosecoordinates are calibrated relative to those of the X-Y stage, obviatingthe need for a reference scan with parallel collimator.

According to another embodiment, the arrangement 10 of FIG. 1A can beimplemented on the SPECT system 50 by mounting the array 14 to thepatient bed 51. The parallel hole collimator and the collimator 8 to bemeasured are positioned in place of the collimator 58 of detector unit53. The motion of the movable patient bed 51 in a SPECT system 50 can beaccurately controlled and thus can be used for the scanning/steppingprocess by moving the array 14 in stepped increments over the detectorunit 52, for example, while maintaining the distance between the array14 and the interaction plane 4 of the detector 6 to the fixed distanceD. The data collected by the detector 6 would be processed and stored inthe control unit 57 so that the processor units 60 can carry out themethods described above for calculating the dx, θx and dy, θy. Theseries of line images such as those shown in FIGS. 3A, 3B would begenerated by the processor units 60 and stored in machine-readablememory units 62.

Accuracy of the system can be increased by an iterative process wherebythe pixel location in the direction orthogonal to the focusing directionis re-computed using the map of the orthogonal direction angles. Detailsof such second order processes will vary with type of focusingcollimation, but are obvious to those skilled in the art.

As noted, the embodiments described herein utilized a variable focallength fan beam collimator 8 as an example but, it will be appreciatedthat the disclosed system and method can be used to measure the holeangles of a variety of collimator types.

FIG. 5 is a flowchart summary of the method of the present disclosureaccording to an embodiment. The method of measuring the hole orientationangles of a collimator comprises providing a plurality of parallelspaced apart line sources 2 mounted on the carrier 14 at distance D fromthe interaction plane 4 of the detector 6. (See box 100). A firstcollimator is then positioned between the detector 6 and the carrier 14.(See box 200). A set of line images of the plurality of line radiationsources is obtained by scanning/stepping the carrier 14 across a firstcollimator in a first direction, wherein the first direction is one ofthe x or y direction in the collimator's x-y coordinate. (See box 300).As described above, the scanning/stepping procedure requires the carrier14 to be stepped across the width of the collimator in steppedincrements. Next, a second set of line images of the plurality of lineradiation sources is obtained by scanning/stepping the carrier 14 acrossthe first collimator in a second direction that is orthogonal to thefirst direction. (See box 400). Thus, if the first direction was alongthe x direction, the second direction would be along the y direction andvice versa. In one embodiment, the scanning/stepping in the seconddirection is carried out by first rotating the carrier 14 90-degrees inthe x-y plane. Next, the procedures of the boxes 300 and 400 arerepeated for a second collimator, wherein one of the two collimators isa reference collimator (e.g. the parallel hole reference collimatordiscussed above) and the other of the two collimators is the measuredcollimator (the collimator being measured, e.g. the variable focallength fan beam collimator 8 discussed above). (See box 500). The lineimage data are analyzed for the reference collimator and the collimatorbeing measured to determine the offset distances dx, dy for each pair oflines between the reference collimator's line image data and themeasured collimator's line image data. (See box 600). Next, the holeorientation angles θx, θy for each of the holes in the measuredcollimator are calculated per formula (1) discussed above. (See box700). The hole orientation angles θx, θy are stored as a vector map inthe firmware associated with the measured collimator.

It will be appreciated that although the disclosed embodiments describescanning/stepping the plurality of line radiation sources 2 with respectto the collimator and detector 6, it is contemplated that the linesources 2 can instead be held stationary and the collimator and detector6 can be moved in stepped increments. The important point is that theline radiation sources 2 and the collimator and the detector 6 can becontrollably stepped across with respect to each other to generate theseries of line images from the collimated radiation impinging on thedetector 6 and that the scan locations of first and second collimatorare repeatable.

It will be appreciated that although the disclosed embodiments describescanning/stepping the plurality of line radiation sources 2 with respectto the collimator to be measured and also with respect to a referencecollimator, the scan with the reference collimator can be obviated bycareful calibration of an accurate X-Y stage with respect to a detectorover which the lines are scanned.

The method for operating the disclosed arrangement, as described herein,may be automated by, for example, tangibly embodying a program ofinstructions upon a machine-readable storage media, such as themachine-readable storage unit 62 of the SPECT system 50, capable ofbeing read by a machine, such as the processor unit 60, capable ofexecuting the instructions. A general purpose computer and/or computerprocessor is one example of such a machine. A non-limiting exemplarylist of appropriate storage media well known in the art would includesuch devices as a readable or writeable CD, flash memory chips (e.g.,thumb drives), various magnetic storage media, and the like. FIG. 6Ashows a system in which actual measured collimator holes are used inaccordance with an embodiment of the present invention. SpecificallyFIG. 6A discloses a schematic of a mathematically perfect collimator 68creating a histogramed image 72 in a detector 70 of a radioactive source66. The diagram also depicts what happen physically with source 66,collimator 68, and detector 70 as well as depicting what happens in aniterative reconstruction algorithm during the reconstruction process.

The elements of FIG. 6A describe what happens physically, when aradioactive source 66 is placed in front of a detector 70 collimated by,for example, a parallel beam collimator. Of the gamma rays emittedisotropically from the source 66, only those travelling in thedirections of the holes, which are perpendicular to detector surface inthe example, are able to pass through the collimator 68 and be detectedin the position sensitive detector 70, yielding the detected counthistogram 72.

The same elements describe what happens in a computer at one step of theiterative tomographic reconstruction process. During tomographicreconstruction the radioactive source 66 represents the current estimateof the voxelized object maintained in computer memory. This activityobject estimate is forward projected onto the detector 70 at anglesorthogonal to the surface (in the parallel collimator example) to form adetected count estimate. The set of orthogonal lines 68 in FIG. 6A canalso represent a vector map look up table (LUT) in computer memorydefining the direction in which to project the activity objectestimate—a trivial one in this case of zero degrees for all detectorpixels. The resulting projection estimate is compared to the detectedcount data, and the difference is used to improve the estimate of theobject. The process is then repeated. Data for the LUT may be gatheredusing the process disclosed in U.S. patent application Ser. No.12/987,376, filed on Jan. 10, 2011.

FIG. 6B shows a system in which the measured collimator holes differfrom the idealized collimator holes in accordance with an embodiment ofthe present invention. Specifically, FIG. 6B depicts what would happenin the case where all the collimator holes do not point in the correctdirection. More specifically FIG. 6B discloses a schematic of animperfect collimator 74 creating a histogramed image 76 in a detector 70of a radioactive source 67. The detected count histogram is shiftedrelative to its position in the “mathematically perfect” system of FIG.6A. If the collimator distortions are not accounted for in thereconstruction process, the iterative correction process will assume,incorrectly, that the activity is coming from a location shown byradioactive source 69. This inconsistency between the ideal model(mathematically perfect collimator) and actual model (imperfectcollimator) leads to distortions and artifacts in the tomographicreconstruction. If, however, the vector map LUT used in thereconstruction forward/back-projections correctly describes thedistorted hole directions, as shown in FIG. 6B, then the results for theestimated data and actual data will be consistent and the tomographicimage quality improved.

Prior art systems use an estimated data model in the tomographicreconstruction which assumes the collimator 68 is mathematically perfectas in FIG. 6A. This results in distortions in the image because thecollimator is rarely ever mathematically perfect. It is more likecollimator 74 in FIG. 6B. Applicant's earlier invention disclosed inU.S. patent application Ser. No. 12/987,376, filed on Jan. 10, 2011provides a means for measuring the angles of the collimator holes. Thus,the LUT will define the directions of the lines of response for forwardprojections taking into account the inaccuries of collimator 74. Anembodiment of the current invention provides a practical means of usingthat information to provide an improved tomographic image. That is, nowthat a means for measuring the collimator holes is provided. The actualmeasurements can be used instead of the estimated angles that assume thecollimator is mathematically perfect.

Implementation of the vector map guided forward projections and backprojection process into tomographic reconstruction should be clear tothose skilled in the art, and many variations are possible (ray-driven,data-driven, inclusion of detector/collimator point response function,attenuation, scatter, etc.). In an embodiment of the invention, ameasured vector map of the actual direction that the collimator is“looking”, however obtained, can be utilized to determine the directionsof the forward-/back-projections during tomographic reconstruction.

The features of the system and method have been disclosed, and furthervariations will be apparent to persons skilled in the art. For instance,the vector map of collimator hole directions may be obtained by othermeans. All such variations are considered to be within the scope of theappended claims. Reference should be made to the appended claims, ratherthan the foregoing specification, as indicating the true scope of thedisclosed method.

The functions and process steps disclosed herein may be performedautomatically or wholly or partially in response to user command. Anactivity (including a step) performed automatically is performed inresponse to executable instruction or device operation without userdirect initiation of the activity.

The disclosed systems and processes are not exclusive. Other systems andprocesses may be derived in accordance with the principles of theinvention to accomplish the same objectives. Although this invention hasbeen described with reference to particular embodiments, it is to beunderstood that the embodiments and variations shown and describedherein are for illustration purposes only. Modifications to the currentdesign may be implemented by those skilled in the art, without departingfrom the scope of the invention. The processes and applications may, inalternative embodiments, be located on one or more (e.g., distributed)processing devices accessing a network linking the elements of thedisclosed system Further, any of the functions and steps provided inFIG. 5 may be implemented in hardware, software or a combination of bothand may reside on one or more processing devices located at any locationof a network linking the elements the disclosed system or another linkednetwork, including the Internet.

Thus, although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the invention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

What is claimed is:
 1. A method for reconstruction of single photonemission computed tomographic images comprising: automatically measuringcollimator hole orientation angles; using said measured collimator inforward and/or backward projection steps in the reconstruction, wherebythe reconstructed tomographic images have improved image resolution andreduced distortion and artifact content.
 2. The method of claim 1,further comprising storing the collimator hole orientation angles θx, θyas a vector map in a firmware associated with the measured collimator.3. The method of claim 1, wherein the measured collimator is a parallelhole collimator.
 4. The method of claim 1, wherein the measuredcollimator being measured is a fan beam collimator.
 5. The method ofclaim 4, wherein the measured collimator is a variable focal length fanbeam collimator.
 6. The method of claim 1, wherein the measuredcollimator is a cone beam collimator.
 7. The method of claim 1, whereinthe measured collimator is a slant hole collimator.
 8. The method ofclaim 1, wherein the measured collimator is a diverging collimator. 9.The method of claim 1, wherein the measured collimator is a convergingcollimator.